record blog activity should use date of posting, not date of last post modification

[magnus78]

When recording blog activity the date used is the post_modified_gmt date. This means if you later correct a typo or something in an old blog posting that blog post will suddenly appear as a seemingly new blog post in the activity stream (since it's date got bumped). It should use post_date_gmt, i.e. use the original blog post date.

[magnus78]

proposed fix

[boonebgorges]

Duplicate of #4303.

See also #3746, #2546

[johnjamesjacoby]

Commented on #2546.